The invention relates to an image detector comprising a surface sensitive to photon radiation and protected from the external environment by a transparent window which is integral with a housing and is placed, with an internal diopter and an external diopter, in front of the said sensitive surface, the latter having a surface quality capable of generating radiation reflections over angles which can amount to several dozens of degrees relative to the direction of mirrored reflection on the said surface.
The invention relates particularly to earth sensors and more particularly to earth sensors having a matrix of photosensitive elements with charge transport (CCD matrix) for geostationary satellites.
Image detectors may be classified into two families: those having vacuum tubes whose sensitive surface exposed to photon radiation is a photocathode and those having semiconductor units organized into strips or into a matrix of image elements (or pixels). In most cases, the photosensitive surface of these detectors requires protection against pollution or against degradation caused by the external environment surrounding the detector, whether this relates to dust in the earth's atmosphere or the components of this atmosphere themselves. This protection consists of a window (or porthole) arranged in an impermeable manner in front of the sensitive surface and transparent to the photon radiation to which the relevant detector is sensitive. The window is formed by a glass of optical quality having an internal diopter and an external diopter. The contrast of the image to be analyzed may be impaired in the presence of a source of strong brightness in the field of vision owing to multiple reflections generated, starting from the sensitive surface itself, by secondary parasitic reflections on the two diopters of the protective window. When points or spots of very strong luminance against a weakly illuminated background are observed, these parasitic reflections generate halos on the photosensitive surface whose levels may be much higher than that of the observed scene on earth, or, in space, than any other light source of lesser strength which one wants to observe and which thus constitutes the useful signal, but which is then drowned in the noise. A typical case in which this technical problem arises is when the image detector is a sensor of the earth's contour on board a geostationary satellite and is formed by a CCD matrix for a radiation range comprising the entire visible radiation and the adjoining infrared. However, the earth must be permanently monitored by the detector for a nominal operation of the latter, while the orbit of the satellite is such that during several months of the year, around each equinox, the sun is in the field of the detector for several hours around midnight local time during each revolution of 24 hours. In this illumination situation the luminescence of the sun is approximately 10.sup.6 to 10.sup.7 times stronger than that of the earth's contour. When no special measures are taken, the image of the luminous signal coming from the sun and deflected by the atmosphere around the earth would disappear in the matrix in the parasitic light coming directly from the sun and reflected first by the matrix itself and then by the two diopters of the protective window.
The matrix CCD TH 7864 from Thompson-CSF could form a good image detector suitable for the exact application cited as the last one above, provided it were possible to dispense with the protective window which closes the housing. It is not possible, however, to do away with this window which has a mainly protective function, while an inert gas such as nitrogen, for example, fills the volume confined between the window and the matrix. The known TH 7864 matrix referred to above comprises a housing to the bottom of which the silicon slice forming the actual detector is glued, which housing is provided with rims surrounding the slice and defining a surface which lies in a plane situated at a distance of 0.84 mm above the sensitive surface of the detector. A window of optical glass of 0.9 mm thickness is glued against this surface, perfectly transparent to visible light. In these conditions, the two parasitic halos generated around the image of the sun on the matrix by the direct reflection of the sunlight on the matrix and subsequently on the two diopters occupy diameters of the order of 100 pixels and 200 pixels, respectively. Supposing that the detector is to cover a field greater than the apparent diameter of the earth, and that the diameter of the sun is of the order of 35 times smaller than that of the earth viewed from a geostionary orbit, the direct image of the sun has a typical diameter of 6 pixels and that of the earth of 200 pixels. Since the detection of the earth must be assured even when the sun is close to it or on its edge, the image of the earth which one wants to measure is to a major extent or completely masked by each of the two parasitic halos.
Devices for reducing blooming effects in image detectors are known. A first measure to achieve this object is to provide the objective which is placed in front of the detector with photochromic lenses. When the detector is a vacuum tube, another solution is to interpose a strip of microchannels in the electron path. When the photocathode of the tube receives a strong, point-shaped illumination, the local saturation properties of the strip of microchannels limit the blooming while still rendering possible the observation of the remaining landscape .